PROJECT SUMMARY Waste solutes cleared by the kidney accumulate in end stage renal disease (ESRD). Current dialysis treatment does not restore the plasma levels of these solutes to normal, and persistently high levels of waste solutes contribute to illness in dialysis patients. Recent studies have shown that many uremic solutes, including some of those retained at the highest levels relative to normal in dialysis patients, are produced by colon microbes. DNA sequencing methods enable us to characterize the colon microbial population, or microbiome, and the processes by which it produces uremic solutes. The proposed studies will employ these methods to characterize the colon microbiome in dialysis patients. They will focus on microbial features associated with the well-characterized colon-derived uremic solutes p- cresol sulfate (PCS) and indoxyl sulfate (IS) and on trimethylamine oxide (TMAO), for which there is emerging evidence of toxicity. The first aim is to characterize the microbiome of hemodialysis patients. 16S rRNA gene sequencing will test the hypothesis that colon microbial diversity is reduced in hemodialysis patients as compared to healthy subjects. These studies will further characterize shifts in microbial taxonomy associated with ESRD and hemodialysis therapy. Computational analysis based on 16S rRNA gene sequencing and shotgun sequencing will be employed to assess the functional capacity of the microbiome. These studies will focus on the representation in the microbiome of genes encoding key enzymes responsible for synthesis of PCS, IS, and TMAO, and also provide data on the representation of other genes. The second aim is to characterize the microbiome of peritoneal dialysis patients. My mentor has shown that PCS and IS are produced at much lower rates in peritoneal dialysis patients than in hemodialysis patients. The proposed studies will exploit this finding to further distinguish features of the microbiome responsible for PCS and IS production. 16S rRNA gene sequencing will reveal whether microbial taxonomy is different in peritoneal and hemodialysis patients. Computational metagenomic reconstruction based on 16S rRNA gene sequencing and metagenomic sequencing will test the hypothesis that genes responsible for synthesis of PCS and IS are less frequently encoded in the microbiome of peritoneal dialysis patients. Additional measurements will reveal whether the microbiome produces lesser amounts of other uremic solutes in peritoneal dialysis patients than in hemodialysis patients. The overall aim is to develop knowledge and expertise which can be employed in future efforts to test whether manipulation of the microbiome can benefit dialysis patients.